In the automobile industry, metal and plastics parts are provided with multi-layer coatings. These serve to protect the substrates from corrosion, but should also exhibit a good decorative effect. They are generally multi-layer coatings based on a primer coat and one or more subsequent coats, e.g. base lacquer coats, covering lacquer coats, which can then be provided with a transparent clear lacquer as a subsequent coating.
Water-thinnable base lacquers based on polyurethane/poly(meth)acrylate polymer hybrids are known.
EP-A-0 297 576 describes the polymerisation of (meth)acrylic monomers in the presence of polyurethane dispersions which have been produced beforehand. After polymerisation is complete, aqueous base lacquers can be produced from the dispersions obtained.
EP-A-0 353 797 describes the production of a hydroxy-functional polyurethane resin in an unsaturated monomer as a solvent, wherein the unsaturated monomer may be reactive towards isocyanate. After conversion of the polyurethane solution into the aqueous phase, an emulsion polymerisation is carried out. The hybrid polymer dispersion which is obtained can be used as a binder vehicle for aqueous base lacquers.
EP-A-0 401 565 describes aqueous base lacquers, the binder vehicles of which are produced by the emulsion polymerisation of aqueous emulsions, which are prepared with the addition of external emulsifiers, of polyurethane macromonomers which have been functionalised to render them unsaturated and which are dissolved in unsaturated monomers.
EP-A-0 424 705, EP-A-0 521 928, EP-A-0 522 419, EP-A-0 522 420, EP-A-0 662 992 and WO 95/14721 describe aqueous base lacquers which are based on polyurethane/poly(meth)acrylate polymer hybrids, wherein the polymer- hybrids have been synthesised by the graft or block polymerisation of unsaturated monomers with polyurethane macromonomers which have been functionalised to render them unsaturated. In this connection, the unsaturated groups of the polyurethane macromonomers may be lateral or terminal groups. The unsaturated polyurethane macromonomer may have been prepared dissolved in an organic solvent, for example in the unsaturated monomer itself, or in the absence of solvents. Copolymerisation can be conducted in the aqueous or non-aqueous phase, namely before or after conversion into the aqueous dispersion.
Aqueous emulsions, particularly those which are free from emulsifiers, of crosslinkable polymer-polyurethane resins which contain hydroxyl groups are known from DE-A-44 13 737. These aqueous dispersions can be obtained by the joint chain extension of polyurethanes, which are free from urea groups and which comprise OH groups which are substantially terminal, together with polymers which contain hydroxyl groups and which are based on ethylenically unsaturated monomers, with diisocyanates in a non-aqueous medium, neutralisation of carboxyl groups which are present and which can be converted into anionic groups, and conversion of the resulting product into the aqueous phase. The aqueous binder vehicle dispersions which are obtained in this manner are very suitable as binder vehicles for aqueous base lacquers such as those which are used in the production of decorative multi-layer coatings in the field of motor vehicle coating.
The known aqueous base lacquers are in need of improvement as regards the stone impact resistance of the multi-layer coatings which are produced with them. If these aqueous base lacquers are used in repair coating operations in the context of mass production coating, for example for what is termed final assembly repair work, the adhesion of the base lacquer coats produced with them to the outer clear lacquer coat, particularly to a clear lacquer coat which has not been sanded, is insufficient for a multi-layer coating.